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rfc:rfc2094

Network Working Group H. Harney Request for Comments: 2094 C. Muckenhirn Category: Experimental SPARTA, Inc.

                                                            July 1997
         Group Key Management Protocol (GKMP) Architecture

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Table of Contents

 1. Introduction.................................................   1
 2. Multicast Key Management Architectures.......................   3
 3. GKMP Protocol Overview.......................................   9
 4. Issues.......................................................  19
 5. Security Considerations......................................  22
 6. Authors' Address.............................................  22

Abstract

 This specification proposes a protocol to create grouped symmetric
 keys and distribute them amongst communicating peers. This protocol
 has the following advantages: 1) virtually invisible to operator, 2)
 no central key distribution site is needed, 3) only group members
 have the key, 4) sender or receiver oriented operation, 5) can make
 use of multicast communications protocols.

1 Introduction

 This document describes an architecture for the management of
 cryptographic keys for multicast communications.  We identify the
 roles and responsibilities of communications system elements in
 accomplishing multicast key management, define security and
 functional requirements of each, and provide a detailed introduction
 to the Group Key Management Protocol (GKMP) which provides the
 ability to create and distribute keys within arbitrary-sized groups
 without the intervention of a global/centralized key manager.  The
 GKMP combines techniques developed for creation of pairwise keys with
 techniques used to distribute keys from a KDC (i.e., symmetric
 encryption of keys) to distribute symmetric key to a group of hosts.

Harney & Muckenhirn Experimental [Page 1] RFC 2094 GKMP Architecture July 1997

1.1 Multicast Communications Environments

 The work leading to this report was primarily concerned with military
 command and control and weapons control systems, these systems tend
 to have top--down, commander--commanded, communications flows.  The
 choice of what parties will be members of a particular communication
 (a multicast group for example) is at the discretion of the "higher"
 level party(ies).  This "sender-initiated" (assuming the higher-level
 party is sending) model maps well to broadcast (as in
 electromagnetic, free-space, transmission) and circuit switched
 communications media (e.g., video teleconferencing, ATM multicast).
 In looking to apply this technology to the Internet, a somewhat
 different model appears to be at work (at least for some portion of
 Internet multicast traffic).  IDRP and Distance Vector Multicast
 Routing Protocol (DVMRP) use multicast as a mechanism for parties to
 relay common information to their peers.  Each party both sends and
 receives information in the multicast channel.  As appropriate, a
 party may choose to leave or join the communication without the
 express permission of any of the other parties (this begs the
 question of meta-authorizations which allow the parties to
 cooperate).  More interestingly, the multicast IP model has the
 receiver telling the network to add it to the distribution for a
 particular multicast address, whether it exists yet or not, and the
 transmitter not being consulted as to the addition of the receiver.
 Other applications of multicast communications in the Internet, for
 example NASA Select broadcasts, can be viewed as implementing the
 sender model since the sender selects the broadcast time, channel,
 and content, though not the destinations.
 It is our intention to provide key management services which support
 both communications (and implied access control) models and operate
 in either a circuit switched or packet switched environment.

1.2 Security for Multicast

 Multicast communications, as with unicast, may require any of the
 security services defined in ISO 7498, access control, data
 confidentiality, traffic confidentiality, integrity/data
 authentication, source authentication, sender and receiver non-
 repudiation and service assurance.  From the perspective of key
 management processes, only data confidentiality, data authentication,
 and source authentication can be supported.  The other services,
 traffic confidentiality, non-repudiation, and service assurance must
 be provided by the communications protocol, they may rely on
 cryptographic services but are not guaranteed by them.

Harney & Muckenhirn Experimental [Page 2] RFC 2094 GKMP Architecture July 1997

2 Multicast Key Management Architectures

2.1 Current Operations

 There are several electronic mechanisms for generating and
 distributing symmetric keys to several computers (i.e.,
 communications groups).  These techniques, generally, rely on a key
 distribution center (KDC) to act as a go between in setting up the
 symmetric key groups.  Military systems, such as BLACKER, STU-
 II/BELLFIELD, and EKMS, and commercial systems, such as X9.17 and
 Kerberos, all operate using dedicated KDCs.  A group key request is
 sent to the KDC via various means (on- or off-line) The KDC acting as
 an access controller decides whether or not the request is proper
 (i.e., all members of a group are cleared to receive all the data on
 a group).  The KDC would then call up each individual member of the
 group and down load the symmetric key.  When each member had the key
 the KDC would notify the requester.  Then secure group communication
 could begin.  While this was certainly faster then anything that
 requires human intervention.  It still requires quite a bit of set-up
 time.  Also, a third party, whose primary interest isn't the
 communication, needs to get involved.
 Pairwise keys can be created autonomously by the host on a network by
 using any number of key generation protocols (FireFly, Diffe-Hellman,
 RSA). These protocols all rely on cooperative key generation
 algorithms to create a cryptographic key.  These algorithms rely on
 random information generated by each host.  These algorithms also
 rely on peer review of permissions to ensure that the communication
 partners are who they claim to be and have authorization to receive
 the information being transmitted.  This peer review process relies
 on a trusted authority assigning permissions to each host in the
 network that wants the ability to create these keys.  The real beauty
 of these pairwise key management protocols is that they can be
 integrated into the communication protocol or the application.  This
 means that the key management becomes relatively invisible to the
 people in the system.

2.2 GKMP-Based Operations

 The GKMP described below, delegates the access control, key
 generation, and distribution functions to the communicating entities
 themselves rather than relying on a third party (KDC) for these
 functions.  As prelude to actually distributing key, a few things
 must be assumed (for purposes of this document): there exists a
 "security manager" responsible for creating and distributing to
 parties authentic identification and security permission information
 (The security manager function may be accomplished through a strictly
 hierarchical system (a la STU-III) or a more ad hoc system of

Harney & Muckenhirn Experimental [Page 3] RFC 2094 GKMP Architecture July 1997

 cooperating peer "domain managers," the implementation of the
 certification hierarchy is not addressed in this document.);
 communicating parties are online for the keys formed and distributed
 by the GKMP.

2.2.1 Sender Initiated Operations

 This section describes the basic operational concept for multicast
 key management for sender initiated multicast support.  This model of
 multicast communications was the basis for our original work on
 multicast key management.  From a security viewpoint the sending
 application is able to control access to the transmission through
 both key distribution and communications distribution (not sending
 the transmission to some addresses).
 Identification of Group Key Controller -- The originator of the
 multicast group creates or obtains a group management certificate
 from its certification hierarchy.  The certificate identifies the
 holder as responsible for generation and distribution of the group
 key (Naming standards are not addressed here, the name should reflect
 the naming structures appropriate for the supported cryptographic
 service.  For example, IP-level encryptors should use naming
 reflecting "host" identities (IP addresses, or DNS host names), RTP
 encryptor would use session names).  The originator relays the
 membership list to the Group Key Management (GKM) application.
 Group Key Creation --   The GKM application, operating on behalf of
 the originator, selects one member of the group, contacts it, and
 creates a Group Key Packet (GKP). A GKP contains the current group
 traffic encrypting key (GTEK) and future group key encrypting key
 (GKEK). The GKM application then identifies itself as the group key
 controller, which the member validates, under cover of the GTEK.
      Group Key Packet (GKP) = [GTEKn,GKEKn+1]
 As part of group key packet formation, usage parameters, appropriate
 for the underlying crypto-system, are selected.  Unlike normal
 parameter negotiation, where common security-level/range, and
 services are arrived at, the originator's GKM application selects
 these parameters and the member must comply.
 Group Key Distribution --   After creation of the GKP, the group
 controller contacts each member of the group, creates a Session Key
 Package (SKP), validates their permissions (check member's
 certificate against group parameters), and create a Group Rekey

Harney & Muckenhirn Experimental [Page 4] RFC 2094 GKMP Architecture July 1997

 Package for that member.  A SKP contains a session TEK and a session
 KEK for a particular member.  A GRP contains the GKP encrypted in a
 KEK and signed using the originator's certificate.
      Session Key Package (SKP) = [STEK, SKEK]
      Group Rekey Package (GRP) = {[GKP]KEK} SignatureController
 Group Rekey --   When the group needs to be rekeyed, the originating
 GKM application selects a member, creates a new GKP, creates a new
 GRP (which is encrypted in the previously distributed next GKEK) and
 broadcasts it to the group.
 This procedure is fairly complex, but other than for the distribution
 of site-specific certificates, no centralized key management
 resources are needed.  The only parties to the key management
 communications are the same parties which will be participating in
 the group.

2.2.2 Receiver Initiated Operations

 This section describes key management operational concept for
 receiver initiated multicast communication support.  The receiver
 initiated model presents some interesting problems from a security
 view point since the end-participants are not known a priori.  Also,
 in a purely receiver initiated application (such as DVMRP), there is
 no concept of an "originator" and the participants in the group may
 be quite dynamic with participants changing on a minute by minute
 basis.
 For secure group communications to take place, all members must
 obtain the same key.  This may be achieved by either using
 deterministic key generation techniques (using a secret, shared seed)
 or by making one member of the group responsible for creation of the
 key.  The use of a deterministic key generator presents security
 problems, particularly regarding loss of the seed (it compromises
 both past and future traffic).  The assignment of a member to the
 role of key "controller" also presents drawbacks, but these relate to
 determining which one should be the controller and the need for each
 member to contact him.  The remainder of this discussion will look at
 how the "controller" concept from above could work in the receiver
 initiated case.
 Selection of Group Key Controller --   A group member will be made
 responsible for initial group establishment and periodic generation
 and dissemination of new GRPs.  There is no need for the selected
 controller to be the controller for all time, but at any one time
 only one controller may be active for each group.  Selection of

Harney & Muckenhirn Experimental [Page 5] RFC 2094 GKMP Architecture July 1997

 controller may be made through a voting system, by a simple default
 (the first to transmit to the group is the controller), or
 configuration.
 The current controller's identity must be made available to all
 members, and potential members, for initial group key load and error
 recovery.  The information may be relayed by broacast on a key
 management "channel," or through a directory service.
 Group Key Creation --   The GKP is created and distributed in much
 the same way as in sender initiated operations.  The controller
 creates a GKP with the first group member to initiate contact.  The
 GKM application then identifies itself as the group key controller,
 which the member validates, under cover of the GTEK. Parameter
 negotiation is performed and the first group member is keyed.
 Group Key Distribution --   After creation of the GKP, as other
 members contact the controller, a SKP is created, member permissions
 are validated and a GRP is loaded to the member.
 For widely distributed groups, a form of distributed dissemination
 may be used.  Some number of regional GKM applications are enabled
 with the ability to validate the permissions of new members and upon
 validation send to them the current GKP.(Access control is not
 defined in this document, but it is assumed that both hierarchical
 and discretionaly (rule-based and identity-based) access control will
 be supported.) These regional key distributors perform the same
 functions as the controller, except that they do not create the GKP.
 This concept can be expanded to the point where all current members
 are capable of downloading the GKP, and passing on that capability.
 Group Rekey --   When the group need rekeying the procedure would be
 identical to the sender initiated case.  The controlling GKM
 application selects a member, creates a new GKP, creates a new GRP
 (which is encrypted in the previously distributed next GKEK) and
 broadcasts it to the group.

2.3 GKMP Features

 This section highlights areas which we believe the GKMP approach has
 advantages over the "traditional" KDC based approaches.

2.3.1 Multicast

 Multicast protocols are a growing area of interest for the Internet.
 The largest benefit of a multicast protocol is the ability of several
 receivers to simultaneously get the same transmission.  If the
 transmission is of a sensitive nature, it should be encrypted.  This

Harney & Muckenhirn Experimental [Page 6] RFC 2094 GKMP Architecture July 1997

 means that the all members of the group must share the same
 encryption key to take benefit of the multicast transmission.
 To date the only way of setting up a group of symmetric keys is with
 the assistance of a centralized key management facility.  This
 facility would act as a key broker creating a distributing key to
 qualified group members.  There are several problems with this
 centralized concept.  These problems give rise to many of the
 following motivations for creating a distributed key management
 protocol.

2.3.2 Increase the autonomy of key groups

 The GKMP proposes to extend the pairwise key paradigm to grouped
 keys.  This protocol can be integrated into the communication
 protocols or applications and can become invisible to the host's
 operator.  We will use peer review to enforce our security policy.
 The GKMP allows any host on a network to create and manage a secure
 group.  Maintenance of these group keys can be performed by the hosts
 interested in the group.  The groups themselves will be relatively
 autonomous.  This simplifies the installation of this technology
 allowing more host to use secure multicast communications.

2.3.3 Latency

 Latency refers to the time to set-up or tear down or to re-key a
 group.  In short this corresponds to the length of time it would take
 to set-up a multicast address.
 The GKMP can allow delegation of group creation authority to any host
 in the network.  In essence, when a host needs a group it will have
 the tools needed to create that group and manage it.  Additionally,
 since the host only needs to create a single group it can concentrate
 on that particular group.
 In the current centralized key distribution approach.  The group must
 be requested from the central site.  The central site would process
 that request in accordance with it's priority and current workload.
 Latencies would develop if the workload of the central site gets
 unwieldy or if the communications to the site become overloaded.

2.3.4 Extendibility

 One of the problems with a centralized key distribution system is the
 concentration of key management workload at a single site.  The
 process of creating key groups -- key creation, access review,
 communication to group members takes time and effort.  As the number

Harney & Muckenhirn Experimental [Page 7] RFC 2094 GKMP Architecture July 1997

 of groups on the network grows and the number of group members group.
 The workload at that central sight quickly reaches capacity.
 GKMP should allow a great number of groups to exist on the Internet
 without overloading any particular host.  Delegation of the net wide
 group creation and management workload places the burden of
 maintaining groups on the hosts interested in using those groups.
 Not only is this more efficient, but it places the burden in an
 appropriate location.
 The GKMP distributes the communication requirements to manage groups
 across the network.  Each group manages the group using the same
 communication resources needed to pass traffic.  It is likely that if
 a communication group can support the traffic of a group, it will be
 able to support the minimal traffic needed to management the keys for
 that group.
 GKMP provides it's own access control, based on signed netwide
 permission certificates.  This partially disseminates the burden of
 access control and permission management.  A system wide authority
 must assign the permission certificates, but day to day access
 control decisions are a GKMP responsibility.

2.3.5 Operating expense

 A centralized key distribution site contains, at one time or another,
 the keys for the net.  This is a valuable target for someone to
 compromise.  To protect this site physical and procedural security
 mechanisms are employed (e.g., guards, fences, intrusion alarms, two
 person safes, no-alone zones).  These mechanisms do not come cheap.
 Allowing the hosts to create and manage their keys eliminates the
 need for an on-line centralized key distribution site.  The protocol
 approach restricts access to the keys to the hosts using them (the
 minimal set).  Since, the encryption mechanisms will have already
 incurred the cost to be physically secured there is no additional
 cost levied on the system by the key management system.

2.3.6 Communication Resources

 Because a centralized site is involved in creating, distributing,
 rekeying, and providing access control for every group, it is
 frequently accessed.  The communication resources available to this
 site often become a bottle neck for the groups.  Therefore a big pipe
 is usually installed to this facility.

Harney & Muckenhirn Experimental [Page 8] RFC 2094 GKMP Architecture July 1997

 The GKMP proposes delegating most of the key creation, distribution,
 rekey and access control mission to the hosts that need the secure
 communication.  There no longer is a single third party that must be
 consulted prior to every group key management action.  Hence, the
 communications requirements to manage the keys have shifted to the
 groups themselves.  The need for special high capacity communications
 has been eliminated.

2.3.7 Reliability

 Delegating key management responsibility to the groups eliminates the
 centralized key management site as a single point of failure.  The
 groups that will use the key are responsible for it.  If the
 communications system fails for the key management it is also down
 for the communications.
 The GKMP will attempt to delegate as many functions to the group as
 possible.  There will be some functions which still need to be
 performed outside of the group (granting of privileges).  These
 functions can still fail.  The GKMP will operate on the old set of
 permissions.  These functions need not be in-line.  They are
 performed separate from the key management actions and are not
 crucial to day-to-day operation.

2.3.8 Security

 People are the most risky element for security.  A distributed
 protocol eliminates many people from the key distribution chain.
 This limits "exposure" of the key.

3 GKMP Protocol Overview

3.1 Supporting functions

 A secure key management protocol needs a number of supporting
 functions, especially in a military environment.  The two major
 support functions are security management and network group
 management.  In the commercial world a company could provide these
 support functions.
 The issue of Security Management is permission management, in a
 military environment separation of data occurs along classical
 classification lines (i.e., TOP SECRET to UNCLASSIFIED). In the
 commercial world these levels are proprietary or need to know access.
 Network group management provides an interface to the communications
 system and control of network resources.  Some entity either a
 commercial or military system, the host or network operations center,

Harney & Muckenhirn Experimental [Page 9] RFC 2094 GKMP Architecture July 1997

 must provide the key management protocol with a list of the group
 members.  Also, if the network resources, bandwidth and processing,
 are considered scarce a management structure must allocate them.

3.1.1 Security management

 Security management is a role performed for the entire network.  It
 involves netwide issues of permission management, initialization of
 software, and compromise recovery.  The GKMP relies on security
 management to operate.  Refer to figure 1:  Security management view.
 The GKMP must assume trusted handling of the protocol software prior
 and during installation.  If the GKMP is to use peer to peer access
 control the system must control the assignment of permissions.  These
 permissions must be monitored and updated as needed.  Finally,
 overview of these permissions must include the maintenance of a
 Certificate Revocation List.
 Secure start-up  We need to control the process of loading GKMP
 software onto a host and initializing it.  The protocol needs keys,
 Security Manager --> --> --> --> --> --> --> --> --> --> --> Network
                                 Permissions
                                 Secure Start-ups
                                 Compromise recovery
                  Figure 1:  Security Management View
 public and private, to operate.  It also must have identify
 information of the host on whose behalf it will act.
 There are some life cycle and security concerns with the software
 while in transit, stored, distributed, and installed.  A one time
 start-up procedure must verify the identity of the host.  Procedural
 and physical identification techniques will verify the identity of
 the host (i.e., the Armed Forces Courier Service (ARFCS) accounting,
 or registered mail).  Upon key delivery the security manager logs
 it's receipt and assumes responsibility for the key.
 After proper installation of the software a paper trail verifies the
 recipient.  The computer would initiate an association with the
 security management function to initialize the protocol software
 (create a unique public and private key pair for network operation
 and receive network permissions).  This activation process uses keys
 distributed with the software (good only for initialization) to
 secure an exchange with the security manager.  The host then creates
 a unique public and private pair and sends the public key to the

Harney & Muckenhirn Experimental [Page 10] RFC 2094 GKMP Architecture July 1997

 security manager.  The security manager creates a credential that
 uniquely identifies the host and it permissions.  This credential is
 signed by the security management with its private key and can be
 verified by all net members with the public key.
 Permission management  Each host on the network is given a
 permissions certificate signed by the security management which
 uniquely identify that host and identifies the access permissions it
 is allowed.  These permission certificates are used by the network
 hosts to assign permissions to other hosts.
 This process assigns permissions to equipment or human beings in
 accordance with their duties.  This process involves security
 clearances and human judgment therefore it is outside the scope of
 this protocol.
 The security management function, especially in military operations,
 would be responsible for managing permissions and classifications at
 each host.  In the commercial world, permission management
 corresponds to projects or duties.
 Compromise recovery management  If a group member is found
 compromised, the protocol must facilitate the exclusion of the
 compromised member and return to secure operations.  The security
 management function will provide control of compromise recovery.
 Usually, physical inspections or accounting techniques find
 compromises.  These separate systems report the compromise to the key
 management system.  We must assume the loss of all key resident at
 that host.  The security management function will rescind the
 permission allocated to this compromised host.  We create a list of
 all know compromised hosts and distribution that list across the
 network.  Each host is then responsible for reviewing the propriety
 of each association and enforcing access control to data.

3.1.2 Group management

 The group manager interacts with other management functions in the
 network to provide the GKMP with group membership lists and group
 relevant commands.  The GKMP deals strictly with cryptographic key.
 It relies on external communication and network management services
 to supply network related information.  Primarily, it relies on the
 network management service to provide it with the addresses of group
 members (if the group is sender initiated).

Harney & Muckenhirn Experimental [Page 11] RFC 2094 GKMP Architecture July 1997

 The GKMP allows an external entity to determine the controller of a
 group.  The controller of the group should be able to handle the
 additional processing and communication requirements associated with
 the role.  If this is not a necessary function given the
 implementation, this assignment of controller duties can be set to
 some automated default.  However, even if defaulted some external
 management entity determines how the role of controller is allocated.
 The group manager can receive group progress reports from the group
 controller.  The GKMP provides a service for the network.  It makes
 sense that someone in the network is interested in the progress of
 this service.  The GKMP can provide progress reports.  It is up to
 the network management to determine the manner and recipient of the
 reports.  Reference figure 2:  Network manager interaction.
 Group Manager --> --> --> --> --> --> --> --> -->Network Manager
         /\
         |
         |       Commands, Role assignments
         |       Group member list, Reports
         |
         \/
 {[Group Controller]     Network}
                Figure 2:  Network Manager Interaction
 Group to member mapping  When the GKMP is implemented in sender
 initiated group establishment mode, a list of group member addresses
 must be provided as part of the group establishment command.  The
 GKMP will use these addresses to contact the group members and create
 the group.
 The creation of groups involves the assignment of a group address,
 update of router databases, and distribution of this group address to
 the group members.  This is a classic function of network management.
 The GKMP group controller would be another recipient of this
 information.
 Protocol role allocation  The Group Management Protocol assigns roles
 to members of a particular group.  These roles are binary one is
 either the control over the group or a member of a group.  Some
 external entity will allocate the identity of the group controller
 and group receiver.  This is a desirable aspect because some
 computers are more capable (i.e., central site, great deal of process
 power available to control a group).  We allow some external entity
 to allocate these roles to individual group members, this is
 important in the military application do to the fact that in a

Harney & Muckenhirn Experimental [Page 12] RFC 2094 GKMP Architecture July 1997

 commercial application the allocating authority and group controller
 may very well always be the same.
 Group key progress reporting  The Group Key Management Protocol has
 to be able to report to somebody.  If we create a group, we should
 report it to group requester.  Contrarily if we are not able to
 Network = {[(Group 1 controller) Group 1 members],
 [(Group 2 controller) Group 2 members],
 [(Group 3 controller) Group 3 members], }
                Figure 3:  Distributed Group Management
 create a group we should report that especially since failure to
 create a group at least as a first study will highly correlate with a
 failure of the underlying communications.  The Group Key Management
 Protocol does not have an ability to fix the underlying
 communications so the communication management function must deal
 with these failures.

3.2 Protocol Roles

 Creation and distribution of grouped key require assignment of roles.
 These identify what functions the individual hosts perform in the
 protocol.  The two primary roles are those of controller and
 receiver.  The controller initiates the creation of the key, forms
 the key distribution messages, and collects acknowledgment of key
 receipt from the receivers.  The receivers wait for a distribution
 message, decrypt, validate, and acknowledge the receipt of new key.
 One of the essential concepts behind the GKMP is delegation of group
 control.  Since each host in the network has the capability to act as
 a group controller, the processing and communication requirements of
 controlling the groups in the network can be distributed equitably
 throughout the network.  This avoids potential single points of
 failure, communication congestion, and processor overloading.  Refer
 to figure 3:  Distributed group management.

3.2.1 Group controller

 The group controller is the a group member with authority to perform
 critical protocol actions (i.e., create key, distribute key, create
 group rekey messages, and report on the progress of these actions).
 All group members have the capability to be a group controller and
 could assume this duty upon assignment.

Harney & Muckenhirn Experimental [Page 13] RFC 2094 GKMP Architecture July 1997

 The group controller helps the cryptographic group reach and maintain
 key synchronization.  A group must operate on the same symmetric
 cryptographic key.  If part of the group loses or inappropriately
 changes it's key, it will not be able to send or receive data to
 another host operating on the correct key.  Therefor, it is important
 that those operations that create or change key are unambiguous and
 controlled (i.e., it would not be appropriate for multiple hosts to
 try to rekey a net simultaneously).

3.2.2 Group receiver

 Simply stated a group receiver is any group member who is not acting
 as the controller.  The group receivers will:  assist the controller
 in creating key, validate the controller authorization to perform
 actions, accept key from the controller, request key from the
 controller, maintain local CRL lists, perform peer review of key
 management actions, and manage local key.

3.3 Scenarios

3.3.1 Group establishment

 The protocol to establish a group of host that share a cryptographic
 key must create a high quality key, verify that all intended
 recipients have permission to join the group, distribute the key to
 all qualified members, and report on the progress.  This process
 consists of two phases:  creation of the key and distribution of the
 key.  Refer to figure 4:  Group Establishment.
 The group establishment process is proceeds in the following manner.
 First, a "create group" command is issued to the group commander.
 The group controller validates the command to ensure it came from an
 authorized commander and the group is within the controller's
 permission range.  Next, the controller creates a key.  Then that key
 is passed to the group members, after they pass the peer to peer
 review process.

Harney & Muckenhirn Experimental [Page 14] RFC 2094 GKMP Architecture July 1997

 Group Controller
         |
         |
         \/      Create group keys
         |--> --> --> --> --> --> -->Group member
         |
         |
         \/      Distribute keys
         |--> --> --> --> --> --> --> Group member
         |
         |
         \/      Distribute keys
         |--> --> --> --> --> --> --> Group member
         |
         |
         \/      Distribute keys
         |--> --> --> --> --> --> --> Group member
                    Figure 4:  Group Establishment
 Validate command  The create group command is signed by the group
 commander ( they may be the same device).  This signature should be
 asymmetric in nature.  The public key to validate this command can be
 sent with the command itself, if the public bound to the identity of
 the commander.
 The group controller receives the command.  It verifies that the
 signature, thereby ensuring the message was sent by the claimed
 source and the message has not been modified in transit.
 Creation of group keys  The controller initiates the creation of two
 keys for use in the group.  The creation of a cryptographic key
 requires that the key be sufficiently random.  Randomizers, capable
 of creating high grade cryptographic key, tend to be hardware based
 and are not likely to be practical for this protocol.  There are
 several established key creation protocols based in software (e.g.,
 Diffe-Hellman, FireFly, RSA). All these software based algorithms
 involve two hosts cooperating to create a cryptographic key.  These
 software algorithms are more appropriate for this protocol.
 Also important, in the creation of these keys, is verification of the
 authorization of the key creation partner.  Authorization to posses
 the keys include permissions that equal or exceed the group traffic
 and identity verification.

Harney & Muckenhirn Experimental [Page 15] RFC 2094 GKMP Architecture July 1997

 Distribution of group keys  The controller distributes the group keys
 to the net members.  The controller must verify the identity and
 permissions of each member prior to the key being distributed.
                         Rekey Group
 Group Controller --> --> --> --> --> -->{Group (group member 1-n)}
                        Figure 5:  Group Rekey
 Likewise, the net member must verify the controller's identity,
 authorization to perform this action, and permissions.
 The key being distributed is the same level as the data that it will
 encrypt.  Hence, we must encrypt the key during distribution.  If no
 suitable key exists between the controller and member, a new key must
 be created.  This new key is cooperatively created between the
 controller and net member in a similar manner as the net keys.
 The controller creates a message for encryption in the key held
 between the controller and member.  This message will include key
 management information and the keys.

3.3.2 Group rekey

 Cryptographic key has a life span.  New key must replace "old" key
 prior to the end of its cryptographic life.  This process is rekey.
 Rekey has the advantage of using an existing cryptographic
 association to distribute key.  Also, there is no requirement to
 verify the identity and authorization for the other members.
 Identify and authorization are assumed.
 A group rekey consists of two stages.  First the Group Controller
 creates new group keys.  Second these "new" keys are sent to the
 Group Members in a multicast message.  Refer to figure 5:  Group
 Rekey.
 Creation of group keys  The controller of the rekey will create the
 new keys in exactly the same manner as used during group
 establishment.

Harney & Muckenhirn Experimental [Page 16] RFC 2094 GKMP Architecture July 1997

 Distribution of group keys  The GKMP creates a message for the group
 address.  This message uses one of the keys distributed during group
 establishment to encrypt the new keys.  It also contains an
 authorization token identifying the controller as the rekey agent and
 new management data.  All members of the group using a multicast
 protocol (if one exists) accept this message.
 The message which rekeys the group encrypts the new keys in the
 existing KEK. Since all group members possess the KEK the entire
 group can decrypt this message.
 The token authorizing the group controller to perform this rekey is
 also included.  This token is asymmetrically signed by the group
 commander.  It uniquely identifies the group controller's authority
 to rekey this group.  It also identifies the group the level of
 traffic and rekey interval.

3.3.3 Deletion

 It is desirable to be able to delete group members for either
 administrative purposes or security reasons.  Administrative deletion
 is the deletion of a trusted group member.  It is possible to confirm
 the deletion of trusted group members.  Security relevant deletion is
 the deletion of an untrusted member.  It assumes that the member is
 ignore all deletion commands.
 Administrative delete  Administrative deletion removes the group keys
 from trusted group members.  This deletion consists of two messages
 the first sends a command to the group encrypted in the groups TEK.
 The command essentially says:  acknowledge receipt and then delete
 group keys.  This command is signed by the group controller to
 prevent unauthorized deletions.
 The acknowledgment message is also encrypted under the group TEK and
 is sent to acknowledge receipt of the command.  We could acknowledge
 accomplishment of the command if the net is willing to accept the
 burden of creating pairwise keys between the exiting group members
 and the group controller.
 Compromise recovery  Compromise recovery is the deletion of untrusted
 group members.  This actually involves the creation of an entirely
 new group, without the untrusted member.  Once the new group is
 created, net operations can be shifted to the new group, effectively
 denying the untrusted member access to the data.

Harney & Muckenhirn Experimental [Page 17] RFC 2094 GKMP Architecture July 1997

 There is always a trade-off between security and continued net
 operations when a member is found to be compromised.  The security
 first position states that if a member is compromised, the group must
 be destroyed and then a new secure group created.  However,
 operational concerns sometimes out weigh the security concerns.  The
 operational position is that the group will continue to operate with
 the compromised member and will shift to a new secure group when it
 becomes available.
 The GKMP does not mandate either position.  However, the speed and
 flexibility of the GKMP does allow a new secure group to be created
 quickly.  Thereby, restricting the potential damage done by a
 compromised member.
 Once a member is found to be compromised, that members certificate is
 added to a Certificate Revocation List (CRL). The CRL is an
 asymmetrically signed piece of data, signed by a security manager.
 The list is made up of compromised resource ID's, a version of the
 CRL, and perhaps an identifier of the security manager.  The CRL is
 accessed every time a new key is negotiated.  If one of the key
 creators is on the CRL the key is destroyed and interaction
 terminated.
 The idea behind a CRL is each host would keep records of all open
 associations and compromised resources.  The host would then make
 sure that it does not have and will not create a secure association
 open with anyone who is on the CRL. The CRL concept of becomes more
 complicated in the case of groups.  This is because it is not
 necessary for every member in the group to know who the other group
 members are.  Hence, a group member does not have sufficient
 information to identify compromised group associations.  The GKMP
 proposes that the group controllers be responsible for reviewing the
 CRL and taking appropriate actions should a group member be
 compromised.
 Another issue with CRLs is the speed that they can be distributed
 across a network.  Every time a key is created the cooperating hosts
 exchange the version number of their current CRL. If the versions do
 not match.  The most current version is passed to the host with the
 old version.  Hence, CRLs propagate when keys are created.  If this
 is infrequently and there is a single CRL insertion point, the list
 may take a few days to move across the net.  The GKMP allows a
 speedier distribution of the CRL.
 The GKMP delegates control of groups to specific group controllers (a
 subset of the network).  These controllers are responsible for
 maintaining the security of the group.  If quicker distribution of
 the CRL were desired, the CRL generator ( security management

Harney & Muckenhirn Experimental [Page 18] RFC 2094 GKMP Architecture July 1997

 function could seed the CRL at these controllers.  Controllers are
 points of key management activity and are logical CRL staging areas.

4 Issues

 What are the unresolved issues with this protocol?

4.1 Access Control

 One interesting issue with a grouped key protocol is access control.
 This is because we are moving away from having humans in the loop or
 having a central authority to check the propriety of the group.
 The group protocol must police itself.  It must ensure that each
 member of a group meets the classic military access control policy (
 i.e., a group member`s classification level must be higher or equal
 to the classification of the group that it's in).
 Is allocation of permissions by a higher authority sufficient to
 provide access control?  Or is a more discretionary mechanism
 necessary?

4.2 MLS

 A GKMP must be capable of operating in a multi-level secure
 environment.  The integration of a key management protocol capable of
 creating keys of several different classifications with an operating
 system capable of operating with multiple classifications in non-
 trivial.
 Classified label standards needed to be incorporated.  The
 classification labels used by the key management protocol should
 coincide with the labels used by the MLS operating system.  These
 interoperability issues need to be addressed.

4.3 Error Conditions

 A group protocol is more complex than a pairwise protocol hence there
 are more possible error conditions.  In a pairwise protocol you have
 two parties; they must communicate between themselves.  It is
 relatively simple to define an take care of all the potential error
 conditions.

Harney & Muckenhirn Experimental [Page 19] RFC 2094 GKMP Architecture July 1997

 One assumption with any group protocol is the underlying internet is,
 to some degree, always broken.  The protocol designer has to assume
 that messages will be delayed or destroyed in transit, all member
 will not receive all multicast messages, and acknowledgment of
 actions may not be delivered.  This assumption is important if a
 protocol uses multicast functions to speed-up actions.
 The protocol must provide recovery mechanisms to allow group members
 to recover from loss of messages.  It must recover in a way that is
 transparent to the host and underlying communications network.
 For example, there is an issue whether or not we can create an
 application layer acknowledgment of multi-cast actions.  The issue
 deals with the required bandwidth that acknowledgment would take up.
 It may be much more friendly to the underlying communications systems
 to have each member identify potential errors and correct them in a
 pairwise manner.  The task of handling error conditions in a key
 management protocol is double difficult because many error conditions
 can be induced error condition (invoked by a third party trying to
 break the security of that system) to retrieve there key that is in
 transit or to block the successful dissemination of a key thereby
 attacking the system security mechanism.

4.4 Commercial vs. Military

 Commercial and military key management differ in many ways.
 Commercial Key management protocols tend to emphasize inter-
 operability, freedom of action, and performance.  Military systems
 tend to emphasize security and control of operations.
 There will be a difference in cryptographic algorithms.  The military
 protocol would certainly use high grade encryption because of
 protecting classified information.  The commercial system would
 probably using algorithms.  and techniques certified for unclassified
 communication systems.  The main difference is in the algorithms
 length and type.
 A military protocol would require more management and structure than
 a commercial one.  The military has always adopted a hierarchical
 communication structure and the commercial system, especially if you
 look at the internet, work mainly by anarchist style.

4.4.1 Algorithm Type

 Another difference between military and commercial key management is
 the type of cryptographic algorithms.  The commercial world uses
 encryption algorithms like DES and in the future Skipjack.  The
 military uses other cryptographic algorithms that differ in key

Harney & Muckenhirn Experimental [Page 20] RFC 2094 GKMP Architecture July 1997

 length and have more restrictions.  An example of this would be the
 identification of ACCORDION, as a military key encryption algorithm
 as used in the EKMS program run by NSA.
 Any experiments with a grouped key management protocol must consider
 the differences between military and commercial algorithms.  The
 commercial algorithms tend to be quicker to implement, run faster,
 involve less processing time, and allows an unclassified experiment.
 However, we must be careful not paint an unrealistic picture of the
 performance of the protocol based on these commercial algorithms.  A
 military algorithm tends to be more cumbersome to process, slow to
 process, require more bandwidth, a lot of unpleasant characteristics
 from the commercial stand point, but allow for a higher grade of
 cryptographic security.  One way of dealing with the disparity
 between algorithms is to use the commercial cryptographic algorithms
 and leave the fields the size used by a comparative DOD cryptographic
 algorithms and insert delays to simulate DOD algorithm processing
 times.

4.4.2 Management Philosophy

 Management for a military network is far more structured than a
 commercial network.  A military network would restrict the creation
 of network groups, the rekeying of those groups, and access to the
 data contained in those groups.  In contrast the commercial world
 would enable any member in the network to create a group and allow
 any other member of the net to join that group.
 The group Key Management Protocol must allow for both these
 architectures i.e., all for very structure command control hierarchy
 and another free form group creation.

4.5 Receiver Initiated Operations

 How do they actually work, what are the performance trades,
 experimentation needed.
 Who is the group leader?
 How do we elect a new leader?
 Will multiple leaders be created?
 Will rule based access control allow fine enough access disgression?

Harney & Muckenhirn Experimental [Page 21] RFC 2094 GKMP Architecture July 1997

 Methods for distributed GKP/GRP dissemination need to be examined.
 This includes:
  o  resolving group identification issues, such as how to notify
     potential members of membership requirements without compromising
     any security-relevant information about the group;
  o  approaches for rapidly identifying GKP/GRP sources must be
     developed, such as a "Key ARP" whereby a new member broadcasts
     into the group a request for key service and existing members
     resolve which will provide service; and,
  o  Security effects of distributing access control decisions must
     also be reviewed.

5 Security Considerations

 This document, in entirety, concerns security.

6 Addresses of Authors

 Hugh Harney
 SPARTA, Inc.
 Secure Systems Engineering Division
 9861 Broken Land Parkway, Suite 300
 Columbia, MD 21046-1170
 United States
 telephone:        +1 410 381 9400 (ext.  203)
 electronic mail:  hh@columbia.sparta.com
 Carl Muckenhirn
 SPARTA, Inc.
 Secure Systems Engineering Division
 9861 Broken Land Parkway, Suite 300
 Columbia, MD 21046-1170
 United States
 telephone:        +1 410 381 9400 (ext.  208)
 electronic mail:  cfm@columbia.sparta.com

Harney & Muckenhirn Experimental [Page 22]

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